There is known a conventional hydraulic continuously variable transmission equipped with a first hydraulic system which discharges and intakes a working oil by reciprocation of a plurality of plungers and a second hydraulic system having an output rotary section which acquires output rotation with abutment on a plurality of plungers. The first and second hydraulic systems of such a hydraulic continuously variable transmission share a cylinder block and the same cylinder block rotates about its longitudinal axis.
The cylinder block is provided with a plurality of first plunger chambers in which a plurality of first plungers in the first hydraulic system are retained, a plurality of second plunger chambers in which a plurality of second plungers in the second hydraulic system are retained, and a hydraulic closing circuit for circulating a working oil between the first and second plunger chambers. The reciprocation of a plurality of distributing valves provided on the cylinder block circulates the working oil between the first and second plunger chambers.
In such a hydraulic continuously variable transmission, conventionally, each distributing valve is arranged in parallel to the axis of the cylinder block and the distal end of the distributing valve is made to abut on a swash plate in order to impart axial reciprocation to each distributing valve.
According to the prior art, as the distal end of the distributing valve is made to abut on the swash plate, the distributing valve reciprocates in the axial direction while each distributing valve makes one turn around the axis of the cylinder block. In such a structure, each distributing valve is pressed against the swash plate, and pressing means, such as a spring, is needed.
Further, as shown in FIG. 21, a cylinder block 311 in the conventional continuously variable transmission is provided with a plurality of plunger holes 312 and valve holes 313 which constitute a hydraulic closing circuit. Plungers 314 and selector valves 315 for causing the working oil to produce a predetermined flow operation in the hydraulic closing circuit are respectively disposed in the plunger holes 312 and the valve holes 313. The plunger holes 312 and the valve holes 313 are laid out around the longitudinal axis of the cylinder block 311 and communicate with one another via a fluid passage 317. First and second hydraulic chambers 318 and 319, formed in a ring shape around the longitudinal axis of the cylinder block 311, are provided side-by-side in the axial direction of the cylinder block 311 and communicate with all the valve holes 313 provided in the cylinder block 311.
The selector valve 315 has first to third land portions 316a to 316c formed with approximately the same diameter as the diameter of the valve hole 313, and is formed into the shape of a spool. As the selector valve 315 reciprocates in the valve hole 313, the fluid passage is changed over in such a way that the working oil flows to the fluid passage 317 (plunger hole 312) and either the first or second hydraulic chamber 318, 319.
Since the fluid passage for the working oil is changed over by reciprocation of the selector valve 315 in the conventional apparatus, there is a position where the working oil is not exchanged between the fluid passage 317 and the valve hole 313 (i.e., the first and second hydraulic chambers 318 and 319) during reciprocation of the selector valve 315. The position of the selector valve 315 at this time is called a seal position. When the selector valve 315 is positioned at the seal position, as shown in FIG. 21, a port 320 which is the merging section of the fluid passage 317 and the valve hole 313 is closed by the second land portion 316b of the selector valve 315. As a result, the exchange of the working oil between the fluid passage 317 and the valve hole 313 (first and second hydraulic chambers 318 and 319) does not take place.
In the case structure where the port 320 is closed merely by the second land portion 316b at the time the selector valve 315 is positioned at the seal position, however, pressure is applied intensively to a part of the outer surface of the second land portion 316b by the working oil staying in the fluid passage 317. As a result, the reciprocation of the selector valve 315 may not be carried out smoothly in the valve hole 313.
As shown in FIG. 22, therefore, one may think of a structure in which a diameter-widened portion 321 is formed by widening the portion of the valve hole 313 that corresponds to the port 320, and the diameter-widened portion 321 is caused to face the second land portion 316b at the time the selector valve 315 is positioned at the seal position. This permits the working oil to stay over the peripheral surface of the second land portion 316b even when the working oil is exchanged between the fluid passage 317 and the valve hole 313 (first and second hydraulic chambers 318 and 319). As a result, the reciprocation of the selector valve 315 is performed smoothly without intensive application of pressure to a part of the outer surface of the second land portion 316b by the working oil remaining in the fluid passage 317.
However, the diameter-widened portion 321 is formed at that portion of the valve hole 313 that corresponds to the port 320 located in the lengthwise middle portion thereof. In the fabrication process for the cylinder block 311, therefore, the following problem arises at the time of forming the diameter-widened portion 321 of the valve hole 313. That is, normally, the valve hole 313 is formed by boring a hole with a predetermined diameter by a drill or the like first, then cutting work is performed on the diameter-widened portion 321. At this time, to form the diameter-widened portion 321, a thin tool with an L shape, for example, should be used and should be inserted through the opening of the valve hole 313 for the cutting work. This requires very difficult work in forming the diameter-widened portion 321 and leads to the problem of an increased number of working steps.
The present invention has been devised in consideration of the above-described situations, and aims at providing a hydraulic continuously variable transmission and a power transmission apparatus which can simplify the structure for reciprocating each distributing valve and forming a valve hole simply and easily.